1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which reconstruct an image using a NMR (nuclear magnetic resonance) signal generated by transmitting a RF (radio frequency) signal with a Larmor frequency into an object and forming a gradient magnetic field inside a magnet for a static magnetic field by a gradient coil, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which allow an image with more satisfactory quality to be obtained by adjusting a distance between a radio frequency coil for receiving the nuclear magnetic resonance signal and a body surface of the object.
2. Description of the Related Art
Conventionally, a magnetic resonance imaging (MRI) apparatus is used as a monitoring apparatus in the medical field.
The magnetic resonance imaging apparatus is an apparatus which reconstructs a tomographic image of an object using an NMR signal generated with excitation by transmitting a RF signal with a Larmor frequency from a RF coil so as to resonate a nuclear spin in the object magnetically and forming a gradient magnetic field changing temporally with a gradient coil on a imaging area of the object set to the inside of a cylindrical magnet for generating a static magnetic field.
In the magnetic resonance imaging apparatus as described above, a local RF coil having a size matched with an imaging area is used as an RF coil receiving NMR signals used for obtaining a tomographic image of a specific part in an object with high sensitivity. For example, a WB (whole-body) RF coil is used imaging a large area having a field of view about 50 cm around. On the other hand, a local RF coil with a size matched to a smaller imaging area, such as a coil for a head, a genicula or a vertebra, is used for receiving MRI signals when an imaging area for a tomogrphic image is limited in advance, i.e. on imaging a part, such as a head, a genicula or a vertebra.
Each local RF coil is optimized for an associated body part. Hence, using a local RF coil specialized for receiving a NMR signal from a specific body part allows a local image to be generated with high sensitivity for each imaged area.
To the contrary, a WB coil allows a larger area to be imaged. However it is difficult to obtain a tomographic image with high sensitivity since the distance from a body surface of an object is farther than that in a case where a local RF coil is used.
On the other hand, a case where it is clinically important image a large area with high sensitivity using local RF coils often occurs. However, it is necessary to use a plurality of local RF coils each specified for imaging each specific part in an object in cases where a large area is imaged using local RF coils. Therefore, it is necessary to take out an object from the bed and reset local RF coils on each change of imaged body part, thereby imposing a burden on an object and an operator. That is, conventional local RF coils are preferable for obtaining a local image of an object while imaging a large area needs complicated operations due to moving of the object and changing to other local RF coils.
In order to solve such a problem, the so called moving-bed method which is conducted by imaging while moving a patient bed was devised for keeping a larger imaging area using a single local RF coil or a limited number of local RF coils (see, for example, Japanese Patent Application (Laid-Open) No. 2002-10992).
FIG. 9 is a diagram explaining a method for imaging a large area by a single local RF coil with moving the bed in a conventional magnetic resonance imaging apparatus.
More specifically, as a magnetic resonance imaging apparatus 1 shown in FIG. 9, a local RF coil unit 3 is arranged in an imaging area formed in a magnet 2 in which a not shown gradient coil unit is built. A large area is imaged by combining tomographic images obtained through imaging over a plurality of times each imaging range S of the local RF coil unit 3 with moving a bed 4 setting an object P.
However, on imaging with the conventional moving-bed method, the distance between the local RF coil unit 3 and the bed 4 is constant without being dependent on a position of the bed 4 as shown in FIG. 9. Therefore, a distance between an object P having an uneven surface and the local RF coil unit 3 varies depending on a position of the bed 4. For example, the distance A1 between the abdominal part of the object P and the local RF coil unit 3 is different from the distance A2 between the leg part of the object P and the local RF coil unit 3.
In other words, a distance between a body surface of an object P and the local RF coil unit 3 is not constant. Hence, the sensitivity of the local RF coil unit 3 becomes uneven, thereby being difficult to obtain a tomographic image with more even sensitivity. This situation leads to reduced quality of a tomographic image.